1. Field of the Invention
The present invention relates to a method and an apparatus of voltage measurement, and more specifically to a method and an apparatus having a high resolution for measuring an individual voltage of a voltage supply among a plurality of voltage supplies connected in series and having a high voltage.
2. Description of the Related Art
An electric motor vehicle has a high voltage supply which is formed with a plurality of batteries (voltage supplies) connected in series. JP, H11-248755, A discloses a flying capacitor method for measuring voltage of each voltage supply of a plurality of batteries.
FIG. 6 shows a conventional circuit of the flying capacitor method of voltage measurement. The circuit of the flying capacitor method includes a first multiplexer 1 and second multiplexer 2 connected to terminals T1-T6 of a high voltage supply V, a bipolar capacitor 3, a sampling switch 4, a microcomputer 7, a reference voltage supply circuit (hereafter Vref circuit) 8, and a buffer filter 9.
The microcomputer 7 has a voltage port Vcc connected to a driving voltage supply +Vcc and input ports A/D1 and A/D2. The microcomputer 7 has an analog/digital (A/D) converter to convert an analog voltage inputted to the input ports A/D1 and A/D2 to a digital value.
The Vref circuit 8 has resistors R1-R3, a first and second switches 8a and 8b, and a reference voltage supply +Vref. Each terminal of the resistor R1 is connected to a switch 4a and the resistor R3, respectively and each terminal of the resistor R2 is connected to a switch 4b and the resistor R3, respectively. The resistors R1-R3 form a divider circuit. One end of the first switch 8a is connected to a junction of the resistors R2 and R3 and another end thereof is connected to the reference voltage supply +Vref, which supplies a high voltage or maximum voltage of a measurement full scale value in the microcomputer 7. One end of the second switch 8b is connected to the junction of the resistors R2 and R3, and another end thereof is connected to the ground, which supplies a low voltage or minimum voltage of the measurement full scale value in the microcomputer 7.
The voltage of the reference voltage supply +Vref is set equal to that of the driving voltage supply +Vcc or set to +AVcc (≦+Vcc))
The buffer filter 9 is connected a junction of the resistors R1 and R3, and the input port A/D1 of the microcomputer 7 for eliminating noise inputted to the input port A/D1.
The process of voltage measurement is explained. First, all switches S1-S6, 4a-4b, 8a-8b are opened. When the switches S1 and S2 of the respective multiplexers 1 and 2 are closed, a voltage supply V1, the terminals T1 and T2, the switches S1 and S2, and the capacitor 3 form a closed circuit so that the voltage supply V1 can charge the capacitor 3.
Then, the switches S1-S2 are opened and the second switch 8b is closed and the switches 4a-4b of the sampling switch 4 are closed for a prescribed period of time to supply a voltage between the terminals of the capacitor 3, or the voltage of the voltage supply V1, to the input ports A/D1 and A/D2 through the resistors R1-R3.
Since the second input port A/D2 is connected to the ground (0V), the voltage of the voltage supply V1 is supplied to the first input port A/D1. The microcomputer 7 calculates an absolute value of the voltage difference between the first and second input ports A/D1 and A/D2, |V(A/D1)-V(A/D2)|, and stores the calculated result as a measured voltage value of the voltage supply V1.
When the switches S2-S3 are closed after all switches are opened, a voltage supply V2, the terminals T2 and T3, the switches S2-S3, and the capacitor 3 form a closed circuit so that the voltage supply V2 can charge the capacitor 3 with reverse polarity in contrast with the case of the voltage supply V1.
Then, the switches S2-S3 are opened and the first switch 8a is closed and the switches 4a-4b of the sampling switch 4 are closed for a prescribed period of time to supply a voltage between the terminals of the capacitor 3, or the voltage of the voltage supply V2, to the input ports A/D1 and A/D2 through the resistors R1-R3.
Since the second input port A/D2 is connected to the reference voltage supply +Vref, the first input port A/D1 is supplied with the voltage of the voltage supply V2 and the reference voltage +Vref (+AVcc). The microcomputer 7 calculates an absolute value of the voltage difference between the first and second input ports A/D1 and A/D2, |V(A/D1)-V(A/D2)|, and stores the calculated result as a measured voltage value of the voltage supply V2.
The same processes are repeated with combinations of the switches S3-S4, S4-S5, S5-S6, and so on to measure the respective voltage of the voltage supplies V3-V5 and store the measured voltage values in the microcomputer 7.
In the measurement described above, the voltages of the odd numbered voltage supplies V1, V3, V5 charge the capacitor 3 and supply the microcomputer 7 with the reverse polarity compared to the even numbered voltage supplies V2, V4. The second input port A/D2 is supplied with the voltage of the reference voltage supply +Vref (+AVcc) or 0 volt as the reference voltage. The voltages of the odd numbered voltage supplies V1, V3, V5 are converted to a full scale of the A/D converter, 0V (min) to +AVcc (max), at the first input port A/D1. The voltages of the even numbered voltage supplies V2, V4 are converted to a full scale of the A/D converter, +AVcc (min) to 0V (max), at the first input port A/D1. FIG. 7 illustrates a manner of the conversion.
The full scale value for the voltage measurement is selected as +AVcc as the high voltage or maximum voltage and 0V as the low voltage or minimum voltage in the microcomputer 7 for the voltage measurement apparatus with the flying capacitor method. The voltages of the odd numbered voltage supplies V1, V3, V5 are detected in the range of 0V to +AVcc and the even numbered voltage supplies V2, V4 are detected in the range of +AVcc to 0V. Accordingly, when the +Vcc is 5V and +AVcc is equal to +Vcc, the odd numbered voltages are in the range of 0-5V and the even numbered voltages are in the range of 5-0V.
The conventional A/D converter disposed in the microcomputer 7 has a limited resolution of 5V full scale/10 bits (resolution 0.005 V) for the voltage measurement apparatus with the flying capacitor method as shown in FIG. 6.
When the voltage supplies V1-V5 each having a voltage of 0-20V is detected with 5V full scale utilizing the conventional A/D converter of the microcomputer 7, it is necessary to divide the voltage of V1-V5 with 4:1 to detect. The voltage dividing of 4:1 degrades the resolution of the A/D converter from 0.005V to 0.02V.